Porous carbon materials and production process thereof, and adsorbents, masks, adsorbing sheets and carriers

ABSTRACT

By a process for producing a porous carbon material from a plant-derived material as a raw material, said process including carbonizing the plant-derived material at 800° C. to 1,400° C. and then applying a treatment with an acid or alkali, a porous carbon material having a value of specific surface area of at least 10 m 2 /g as measured by the nitrogen BET method, a silicon content of at most 1 wt % and a pore volume of at least 0.1 cm 3 /g is obtainable from a plant-derived material, which has a silicon content of at least 10 wt %, as a raw material. Also provided is a process for producing a porous carbon material equipped with excellent functionality so that the porous carbon material can be used, for example, as an anode material for batteries, an adsorbent, masks, adsorbing sheets, or carriers.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a national phase entry under 35 U.S.C. § 371of International Application No. PCT/JP2008/056752 filed Apr. 4, 2008,published on Oct. 16, 2008, which claims priority from Japanese PatentApplication Nos. JP 2007-098421 and JP 2008-059052, filed Apr. 4, 2007and Mar. 3, 2008, respectively, in the Japanese Patent Office, all ofwhich are incorporated herein by reference.

BACKGROUND OF THE INVENTION Technical Field

This invention relates to porous carbon materials making use ofplant-derived materials as raw materials and their production process,and also to adsorbents, masks, adsorbing sheets and carriers.

Background Art

In recent years, portable electronic equipment such as camera-integratedVTRs (Video Tape Recorders), digital still cameras, cell phones,portable information terminals and notebook-size personal computers arewidely prevalent, and their size reduction, weight reduction and lifeprolongation are strongly desired. Keeping in step with this desire,developments are under way for batteries, especially secondary batterieshaving light weight and permitting the provision of high energy densityas power supplies for portable electronic equipment.

Among these, secondary batteries, which employ lithium as an electrodereactant in a charge-discharge reaction and use its occlusion andrelease (so-called lithium ion secondary batteries), are highlyprospective because they provide large energy density compared with leadbatteries and nickel-cadmium batteries. In these lithium ion secondarybatteries, carbon materials are extensively used as anode activematerials at anodes (see, for example, Japanese Patent Laid-Open No. Sho62-090863). Employed as such carbon materials to keep production costslow and to provide improved battery characteristics are, for example,cokes such as pitch coke, needle coke and petroleum coke and bakedproducts of organic high-molecular compounds as obtained by baking andcarbonizing furan resins, natural high-molecular materials and the likeat appropriate temperatures (see, for example, Japanese Patent Laid-OpenNo. Hei 4-308670). Further, a technology that bakes an organichigh-molecular compound to produce a porous carbon material havingthree-dimensional regularity is disclosed, for example, in JapanesePatent Laid-Open No. 2005-262324, which also suggests that the porouscarbon material is usable as an anode active material.

Further, carbon materials obtained by baking crystalline celluloses arealso used as anode active materials as such crystalline celluloses varyless in crystalline degree in comparison with synthesized high-molecularcompounds (see, for example, Japanese Patent Laid-Open No. Hei2-054866). These carbon materials can obtain large charge capacitycompared with cokes. In the carbon materials obtained by baking thecrystalline celluloses, however, occluded lithium ions are not readilyreleasable. High charging efficiency is, therefore, hardly available,resulting in the observation of a tendency that as a whole battery, theenergy density becomes smaller.

Unused parts of plants such as vegetables and cereals are mostlydiscarded. Effective utilization of these unused parts is, however,strongly demanded for the preservation and improvement of the globalenvironment. As one example of the effective utilization of such unusedparts, carbonization treatment can be mentioned. It is also investigatedto use, as an anode active material for lithium ion secondary batteries,a carbon material produced by carbonizing such a plant-derived material(see, for example, Japanese Patent No. 3565994, Japanese Patent No.3719790, and PCT Patent Publication No. WO 96/27911).

Further, for patients suffering from a liver disease or kidney disease,the elimination of toxic substances by hemodialysis is performed.However, hemodialysis requires a special system and an experttechnician, and moreover, gives physical and/or mental discomfort orpain to patients. Under such a background, oral adsorbents made ofactivated carbon and having high safety and stability to the body, suchas KREMEZIN, are attracting attention (see Japanese Patent PublicationNo. Sho 62-11611). In addition, anti-obesity agents, antidiabeticagents, anti-inflammatory bowel disease agents, purine adsorbents andthe like, which make use of activated carbon, have also been proposed.Applications, research and developments of activated carbon in themedical field are extensively under way.

In addition, to make a drug to effectively act in the body, it isdesired to allow an appropriate amount of the drug to act for anadequate time. For this purpose, it is preferred to use a carrier thatcan control the release rate of the drug. Adsorption of the drug on sucha carrier makes it possible to continuously release the drug at apredetermined constant level. Such a drug-carrier complex can be used,for example, as a transdermal preparation having transdermalabsorbability and topical action that deliver the drug through the skin,or as an oral preparation. The carrier is composed, for example, ofcarbon having nontoxicity and chemical resistance, an inorganic materialsuch as alumina or silica, or an organic material such as cellulose orpolyethylene oxide. In recent years, however, some examples making useof carbon materials as carriers have been reported (see, for example,Japanese Patent Laid-Open No. 2005-343885). There are also reports onthe sustained release of fertilizer by the use of activated carbon (see,for example, Japanese Patent No. 3694305).

SUMMARY OF THE INVENTION

Nonetheless, the technologies for subjecting plant-derived materials tocarbonization treatment are not considered to be sufficient, and forproduced carbon materials, still further improvements are desired infunctionality. When plant-derived carbon materials are used inelectrochemical devices such as batteries like lithium ion secondarybatteries or electric double layer capacitors, however, sufficientcharacteristics are hardly considered to be obtainable. There is,accordingly, an outstanding strong desire for a carbon material capableof exhibiting still better characteristics when employed inelectrochemical devices. There are also outstanding strong desires forthe development of porous carbon materials for adsorbents orallyadministrable in kidney diseases and liver diseases, the development ofporous carbon materials for the purpose of adsorption of a protein orvirus that gives deleterious effects to the body or as medical adsorbentpreparations having still better adsorption performance, and thedevelopment of porous carbon materials suited for use as carrierscapable of adequately controlling the drug release rate.

An object of the present invention is, therefore, to provide a porouscarbon material, which has high functionality, can obtain excellentcharacteristics when used, for example, in an electrochemical devicesuch as a battery like a lithium ion secondary battery or an electricdouble layer capacitor, and is excellent in adsorption performance anddrug release performance, and its production process, and an adsorbent,a mask, an adsorbing sheet and a carrier.

A porous carbon material according to the present invention to achievethe above-described objects, is obtainable from a plant-derived materialhaving a silicon (Si) content of at least 5 wt % as a raw material, andhas a value of specific surface area of at least 10 m²/g as measured bythe nitrogen BET method, a silicon (Si) content of at most 1 wt % and apore volume of at least 0.1 cm³/g as measured by the BJH method and MPmethod.

In the porous carbon material according to the present invention, it maybe preferred, but is not limited to, that the content of magnesium (Mg)is at least 0.01 wt % but at most 3 wt %, the content of potassium (K)is at least 0.01 wt % but at most 3 wt %, and the content of calcium(Ca) is at least 0.05 wt % but at most 3 wt %.

A process according to the present invention for producing a porouscarbon material from a plant-derived material as a raw material toachieve the above-described object comprises carbonizing theplant-derived material at 800° C. to 1,400° C. and then applying atreatment with an acid or alkali.

The term “carbonizing” or “carbonization” as used herein generally meansto subject an organic substance (a “plant-derived material” in thepresent invention) to heat treatment to convert it into a carbonaceousmaterial (see, for example, JIS M 0104-1984). It is to be noted that asan atmosphere for carbonization, an oxygen-free atmosphere can bementioned. Specifically, a vacuum atmosphere, an inert gas atmospheresuch as nitrogen gas or argon gas, or an atmosphere that brings aplant-derived material into such a form as if it has been roasted in acovered pan can be mentioned. As a ramp-up rate to a carbonizationtemperature, 1° C./min or higher, preferably 3° C./min or higher, morepreferably 5° C./min or higher, each under such an atmosphere, can bementioned. As an upper limit to the carbonization time, on the otherhand, 10 hours, preferably 7 hours, more preferably 5 hours can bementioned, although the upper limit is not limited to such time. A lowerlimit to the carbonization time can be the time in which theplant-derived material can be surely carbonized. Further, theplant-derived material may be ground into a desired particle size asdesired, and in addition, classification may be conducted. Furthermore,the plant-derived material may be washed beforehand.

In the process according to the present invention for the production ofthe porous carbon material, the process may be practiced in such anembodiment that activation treatment is applied subsequent to thetreatment with the acid or alkali. This embodiment can increasemicropores (to be described subsequently herein) the pore sizes of whichare smaller than 2 nm. As an activation treatment method, a gasactivation method or a chemical activation method can be mentioned. Theterm “gas activation method” as used herein means a method that oxygen,steam, carbon dioxide gas, air or the like is used as an activator and aporous carbon material is heated in such an atmosphere at 700° C. to1,000° C. for several tens minutes to several hours to develop amicrostructure by volatile components and carbon molecules in the porouscarbon material. It is to be noted that the heating temperature may bechosen as desired based on the kind of the plant-derived material, thekind and concentration of the gas, and so on. Preferably, however, theheating temperature may be at least 800° C. but at most 950° C. The term“chemical activation method” as used herein, on the other hand, means amethod that activation is effected using zinc chloride, iron chloride,calcium phosphate, calcium hydroxide, magnesium carbonate, potassiumcarbonate, sulfuric acid or the like in place of oxygen or steamemployed in the gas activation method, washing is conducted withhydrochloric acid, the pH is adjusted with an aqueous alkaline solution,and drying is performed.

In the process according to the present invention, including theabove-described preferred embodiments, for the production for the porouscarbon material, silicon components in the plant-derived material afterits carbonization are removed by the treatment with the acid or alkali.Here, as the silicon components, oxidized silicon compounds such assilicon dioxides, silicon oxide and silicon oxide salts can bementioned.

In the process according to the present invention, including theabove-described preferred embodiments and features, for the productionfor the porous carbon material, the process may feature that the contentof silicon (Si) in the plant-derived material is at least 5 wt % andthat in the porous carbon material, the value of specific surface areais at least 10 m²/g as measured by the nitrogen BET method, the contentof silicon (Si) is at most 1 wt %, and the volume of pores is at least0.1 cm³/g as measured by the BJH method and MP method. Further, in theprocess according to the present invention, including these preferredembodiments and features, for the production for the porous carbonmaterial, it may be preferred that in the porous carbon material, thecontent of magnesium (Mg) is at least 0.01 wt % but at most 3 wt %, thecontent of potassium (K) is at least 0.01 wt % but at most 3 wt %, andthe content of calcium (Ca) is at least 0.05 wt % but at most 3 wt %.Furthermore, in the process according to the present invention,including the above-described preferred embodiments and features, forthe production for the porous carbon material, it may be preferred toapply a heat treatment (precarbonization treatment) to the plant-derivedmaterial at a temperature (for example, 400° C. to 700° C.) lower than atemperature for the carbonization in an oxygen-free state before thecarbonization of the plant-derived material, although it depends on theplant-derived material employed. By the heat treatment, tar componentswhich may be formed in the course of the carbonization can be extracted,and as a result, the tar components which may be formed in the course ofthe carbonization can be decreased or eliminated. It is to be noted thatthe oxygen-free state can be achieved, for example, by using an inertgas atmosphere such as nitrogen gas or argon gas, a vacuum atmosphere,or an atmosphere that brings a plant-derived material into such a formas if it has been roasted in a covered pan. Further, the plant-derivedmaterial may preferably be immersed, before its carbonization, in analcohol (for example, methyl alcohol, ethyl alcohol or isopropylalcohol) to decrease mineral components and water contained in theplant-derived material or to avoid the occurrence of an unpleasant smellin the course of the carbonization, although it depends on theplant-derived material to be employed. It is to be noted that theprecarbonization treatment may be conducted after the immersion. Asmaterials to which heat treatment may preferably be applied in an inertgas, plants that abundantly produce pyrolignous acid (tar andlight-weight oil fractions) can be mentioned, for example. As materialsto which the alcohol pretreatment may preferably be applied, on theother hand, seaweeds that abundantly contain iodine and various mineralscan be mentioned, for example.

In the porous carbon material and its production process according tothe present invention, including the above-described various preferredembodiments and features, grain husk or straw of rice (rice plant),barley, wheat, rye, barnyardgrass or foxtail millet, reed, or seaweedstem can be mentioned as the plant-derived material. However, theplant-derived material is not limited to such materials, and in additionto them, vascular plants, ferns and mosses which grow on land, algae,and seaweeds can also be mentioned, for example. It is to be noted thatas a raw material, these materials may be used singly or plural ones ofthem may be used in combination. Further, no particular limitation isimposed on the shape or form of the plant-derived material. For example,grain husk or straw may be used as it is, or may be used in the form ofa dried product. It is also possible to use one subjected to one or moreof various treatments such as fermentation processing, roastingprocessing and extraction processing in the processing of food orbeverage such as beer or liquor. Especially from the viewpoint ofachieving the recycling of an industrial waste as a resource, it ispreferred to use straw or grain hull after processing such as threshing.Straw or grain hull after such processing can be obtained in a largevolume and with ease, for example, from agricultural cooperatives,brewing companies or food companies.

The porous carbon material and its production process according to thepresent invention, including the above-described various preferredembodiments and features, will collectively be called simply “thepresent invention” in the following description. Further, the porouscarbon material according to the present invention, including theabove-described various preferred embodiments and features, and theporous carbon materials obtained by the production process according tothe present invention will collectively be called simply “the porouscarbon material according to the present invention.” Furthermore, thematerial obtained after carbonizing the plant-derived material at 800°C. to 1,400° C. but before applying the acid or alkali treatment will becalled “the porous carbon material precursor” or “the carbonaceousmaterial.”

In the porous carbon material according to the present invention,nonmetal elements such as phosphorus (P) and sulfur (S) and metalelements such as transition elements may be contained. The content ofphosphorus (P) can be at least 0.01 wt % but at most 3 wt %, while thecontent of sulfur (S) can be at least 0.01 wt % but at most 3 wt %. Itis to be noted that the lower the contents of these elements and theabove-mentioned magnesium (Mg), potassium (K) and calcium (Ca), the morepreferred from the viewpoint of an increase in the value of specificsurface area, although it differs depending on the application of theporous carbon material. Needless to mention, the porous carbon materialmay also contain one or more elements other than the above-describedelements, and the above-described content ranges of the various elementsmay vary depending on the application of the porous carbon material.

In the present invention, the analysis of various elements can beperformed by energy dispersion spectroscopy (EDS) while using, forexample, an energy dispersive X-ray analyzer (for example, JED-2200Fmanufactured by JEOL Ltd. (trademark)). As measurement conditions, thescanning voltage and irradiation current can be set, for example, at 15kV and 13 μA, respectively.

When used in electrochemical devices such as batteries like lithium ionsecondary batteries and electric double layer capacitors, phosphorus (P)and sulfur (S) may preferably be contained in the porous carbon materialfrom the viewpoint of improving characteristics such as capacity andcycling characteristics. When the porous carbon material according tothe present invention is used as an anode active material in a lithiumion secondary battery, for example, the inclusion of phosphorus (P) inthe porous carbon material can obtain a high lithium doping level. As aresult, an improvement can be achieved in battery capacity. On the otherhand, the inclusion of sulfur (S) in the porous carbon materialaccording to the present invention can suppress the decomposition of anelectrolyte solution, thereby making it possible to achieve improvementsin cycling characteristics and high-temperature characteristics.

The porous carbon material according to the present invention can beused to selectively adsorb various unnecessary molecules in the body.Therefore, the porous carbon material according to the present inventioncan be used as orally-administrable adsorbent preparations or medicaladsorbent preparations for medical internal medicines or the like usefulfor the treatment and prevention of diseases. When the porous carbonmaterial according to the present invention is applied to the field oforally-administrable adsorbent preparations or medical adsorbentpreparations, specific examples of the adsorbent according to thepresent invention can include an adsorbent for adsorbing creatinine, anadsorbent for adsorbing alizarin cyanine green, an adsorbent foradsorbing lysozyme, an adsorbent for adsorbing albumin, and an adsorbentfor adsorbing an organic substance (for example, an organic molecule orprotein) having a number average molecular weight of 1×10³ to 1×10⁴, allof which comprise the porous carbon material according to the presentinvention. Further, the porous carbon material according to the presentinvention can also be used as a packing material (adsorbent) for bloodpurification columns. In addition, the porous carbon material accordingto the present invention can also be applied as an adsorbent in variousmasks such as, for example, anti-pollinosis masks, and can adsorb, forexample, proteins. Namely, the mask according to the present inventioncan be designed in a form provided with an adsorbent which comprises theporous carbon material according to the present invention. Furthermore,the porous carbon material according to the present invention can alsobe applied to adsorbing sheets. Namely, the adsorbing sheet according tothe present invention can be designed in a form comprising asheet-shaped member, which comprises the porous carbon materialaccording to the present invention, and a support member supporting thesheet-shaped member thereon. Still furthermore, the porous carbonmaterial according to the present invention can also be used as awater-purifying adsorbent for purifying water. It is to be noted that achemical treatment or molecule modification may be applied to thesurfaces of the porous carbon material according to the presentinvention. The chemical treatment can be, for example, a treatment thatforms carboxyl groups on the surfaces by treatment with nitric acid. Byconducting a similar treatment as the activation treatment with steam,oxygen, an alkali or the like, various functional groups such ashydroxyl groups, carboxyl groups, ketone groups or ester groups can beformed in the surfaces of the porous carbon material. Moreover, amolecule modification is also feasible by chemically reacting the porouscarbon material with a chemical species or protein having one or morereactable hydroxyl groups, carboxyl groups, amino groups and/or thelike.

The carrier according to the present invention for carrying a drugthereon can be formed from the porous carbon material according to thepresent invention. Described specifically, when the porous carbonmaterial according to the present invention is assumed to amount to onehundred parts by weight, a complex capable of releasing the drug (adrug-carrier complex capable of adequately controlling the release rateof the drug) can be obtained by adsorbing and carrying one parts byweight to two hundred parts by weight of the drug on the porous carbonmaterial according to the present invention. Such a drug-carrier complex(drug release preparation) comprises the porous carbon materialaccording to the present invention and the drug, and can take such aform that as the weight ratio of the porous carbon material to the drug,the drug amounts to one parts by weight to two hundred parts by weightwhen the porous carbon material according to the present invention isassumed to amount to one hundred parts by weight.

As drugs which can be adsorbed and carried on the porous carbon materialaccording to the present invention, organic molecules, polymer moleculesand proteins can be mentioned. Specific examples can include, but arenot limited to, pentoxifylline, prazosin, acyclovir, nifedipine,diltiazem, naproxen, ibuprofen, flurbiprofen, ketoprofen, fenoprofen,indomethacin, diclofenac, fentiazac, estradiol valerate, metoprolol,sulpiride, captopril, cimetidine, zidovudine, nicardipine, terfenadine,atenolol, salbutamol, carbamazepine, ranitidine, enalapril, simvastatin,fluoxetine, alprazolam, famotidine, ganciclovir, famciclovir,spironolactone, 5-asa, quinidine, perindopril, morphine, pentazocine,paracetamol, omeprazole, metoclopramide, aspirin, and metformin; andfrom the viewpoint of systemic and topical treatments, various hormones(for example, insulin, estradiol, and the like), asthma remedies (forexample, albuterol), tuberculosis remedies (for example, rifampicin,ethambutol, streptomycin, isoniazid, pyrazinamide, and the like), cancerremedies (for example, cisplatin, carboplatin, Adriamycin, 5-FU,paclitaxel, and the like), and hypertension remedies (for example,clonidine, prazosin, propranolol, labetalol, bunitrolol, reserpine,nifedipine, furosemide, and the like). A porous carbon material-drugcomplex can be obtained by dissolving such a drug in an organic solventwhich can dissolve the drug, immersing the porous carbon materialaccording to the present invention in the resultant solution, and theneliminating the solvent and any extra solute. Specific solvents caninclude water, methyl alcohol, ethyl alcohol, isopropyl alcohol, butylalcohol, acetone, ethyl acetate, chloroform, 2-chloromethane,1-chloromethane, hexane, tetrahydrofuran, pyridine, and the like.

The porous carbon material according to the present invention hasnumerous pores. Contained as pores are “mesopores” the pore sizes ofwhich range from 2 nm to 50 nm and “micropores” the pore sizes of whichare smaller than 2 nm. Specifically, pores the pore sizes of which are,for example, 20 nm and smaller are contained numerously as mesopores,with pores the pore sizes of which are 10 nm and smaller being containedparticularly numerously. As micropores, on the other hand, pores of 1.9nm or so in pore size, pores of 1.5 nm or so in pore size and pores offrom 0.8 nm to 1 nm or so in pore size are contained numerously. In theporous carbon material according to the present invention, the pore sizeas measured by BJH method and MP method is at least 0.1 cm³/g, with atleast 0.3 cm³/g being still more preferred.

In the porous carbon material according to the present invention, it isdesired that the value of specific surface area as measured by thenitrogen BET method (which may hereinafter be called simply “the valueof specific surface area”) may be preferably at least 50 m²/g, morepreferably at least 100 m²/g to obtain still better functionality. Whenthe porous carbon material according to the present invention is used inan electrochemical device such as a battery (nonaqueous electrolytesecondary battery) like a lithium ion secondary battery or an electricdouble layer capacitor, for example, the area of an electric doublelayer to be formed at an interface between the porous carbon materialand an electrolyte solution upon charging or discharging becomessufficiently large by controlling the value of its specific surface areato 10 m²/g or greater, preferably 50 m²/g or greater, more preferably100 m²/g or greater so that a large capacity can be obtained. When thevalue of its specific surface area is controlled to 1,500 m²/g orsmaller, on the other hand, an irreversible electrochemical reaction byfunctional groups existing on the surfaces of the porous carbon materialcan be suppressed, and as a result, the charge-discharge efficiency canbe improved.

The term “the nitrogen BET method” means a method that measures anadsorption isotherm by adsorbing nitrogen as adsorbed molecules on anadsorbent (the porous carbon material in this context), desorbing itfrom the adsorbent, and analyzing measured data on the basis of a BETequation represented by the equation (1), and based on this method, aspecific surface area, pore volume and the like can be calculated.Specifically, upon calculation of a value of specific surface area bythe nitrogen BET method, an adsorption isotherm is first obtained byadsorbing nitrogen as adsorbed molecules on an adsorbent (the porouscarbon material) and then desorbing it from the adsorbent. From thethus-obtained adsorption isotherm, [p/{V_(a)(p₀−p)}] are calculatedbased on the equation (1) or an equation (1′) derived by modifying theequation (1), and are plotted against the equilibrium relative pressure(p/p₀). Assuming these plots as a straight line, a slope s(=[(C−1)/(C·V_(m))]) and an intercept i (=[1/(C·V_(m))]) are thencalculated based on the method of least squares. From thethus-determined slope s and intercept i, V_(m) and C are calculatedbased on the equation (2-1) and the equation (2-2). Further, from V_(m),a specific surface area a_(sBET) is then calculated based on theequation (3) (see, pages 62 to 66 of the Analysis Software Manual forBELSORP-mini and BELSORP manufactured by BEL Japan Inc.). It is to benoted that this nitrogen BET method is a measurement method whichfollows JIS R 1626-1996 “Measurement method of the specific surface areaof fine ceramic powder by the gas adsorption BET method.”V _(a)=(V _(m) ·C·p)/[p ₀ −p){1+(C−1)(p/p ₀)}]  (1)[p/{V _(a)(p ₀ −p)}]=[(C−1)/(C·V _(m))](p/p ₀)+[1/C·V _(m))]  (1′)V _(m)=1/(s+i)  (2-1)C=(s/i)+1  (2-2)a _(sBET)=(V _(m) ·L·σ)/22414  (3)whereV_(a): Adsorbed volumeV_(m): Adsorbed volume in a monomolecular layerp: Pressure of nitrogen at equilibriump₀: Saturation vapor pressure of nitrogenL: Avogadro's numberσ: Absorption cross-sectional area of nitrogen

When calculating a pore volume V_(p) by the nitrogen BET method, anadsorbed volume V at a relative pressure preset as a relative pressurefor the calculation of pore volume is determined, for example, bylinearly interpolating the adsorption data of a determined adsorptionisotherm. From this adsorbed volume V, the pore volume V_(p) can becalculated based on the equation (4) (see pages 62-65 of the AnalysisSoftware Manual for BELSORP-mini and BELSORP manufactured by BEL Japan,Inc.). It is to be noted that a pore volume based on the nitrogen BETmethod may hereinafter be called simply “a pore volume.”V _(p)=(V/22414)×(M _(g)/ρ_(g))  (4)whereV: Adsorbed amount at a relative pressureM_(g): Molecular weight of nitrogenρ_(g): Density of nitrogen

The pore sizes of mesopores can be calculated, for example, as adistribution of pores from a rate of change in pore volume relative tothe pore sizes on the basis of the BJH method. The BJH method is amethod widely employed as a pore distribution analysis method. Uponconducting a pore distribution analysis on the basis of the BJH method,a desorption isotherm is first determined by causing adsorption anddesorption of nitrogen as adsorptive molecules on an adsorbent (porouscarbon material). Based on the thus-determined desorption isotherm, thethickness of an adsorbed layer upon stepwise desorption of adsorptivemolecules (for example, nitrogen) from a state that a pore is filledwith the adsorptive molecules and the inner diameter of a pore (twotimes of the radius of the core) formed upon desorption are determined.The pore radius r_(p) is calculated based on the equation (5), and thepore volume is calculated based on the equation (6). A pore distributioncurve can then be obtained from the pore radius and pore volume byplotting the rate of change in pore volume (dV_(p)/dr_(p)) against thepore size (2r_(p)) (see pages 85-88 of the Analysis Software Manual forBELSORP-mini and BELSORP manufactured by BEL Japan, Inc.).r _(p) =t+r _(k)  (5)V _(pn) =R _(n) ·dV _(n) −R _(n) ·dt _(n) ·c·ΣA _(pj)  (6)where R _(n) =r _(pn) ²/(r _(kn)−1+dt _(n))²  (7)wherer_(p): Pore radiusr_(k): Core radius (inner diameter/2) when an adsorption layer of t inthickness is adsorbed at the pressure on the inner wall of a pore havinga pore radius r_(p)V_(pn): pore volume when the n^(th) desorption of nitrogen has occurreddV_(n): Rate of a change upon occurrence of the desorptiondt_(n): Rate of a change in the thickness t_(n) of adsorption layer whenthe n^(th) desorption of nitrogen has occurredr_(kn): core radius at the time of the n^(th) desorption of nitrogenc: fixed valuer_(pn): Pore radius when the n^(th) desorption of nitrogen has occurred.Further, ΣA_(pj) represents the integrated value of the areas of thewalls of pores from j=1 to j=n−1.

The pore sizes of micropores can be calculated, for example, as adistribution of pores from a rate of change in pore volume relative tothe pore sizes on the basis of the MP method. Upon conducting a poredistribution analysis on the basis of the MP method, an adsorptionisotherm is first determined by causing adsorption of nitrogen on anadsorbent (porous carbon material). This adsorption isotherm is thenconverted into a pore volume against the thickness t of the adsorbedlayer (plotted against t). Based on the curvature of the plots (therates of changes in pore volume relative to the rates of changes in thethickness t of the adsorbed layer), a pore size distribution curve canthen be obtained (see pages 72-73 and 82 of the Analysis Software Manualfor BELSORP-mini and BELSORP manufactured by BEL Japan Inc.).

The porous carbon material precursor is treated with an acid or alkali.As a specific treatment method, a method that immerses the porous carbonmaterial precursor in an aqueous solution of the acid or alkali or amethod that reacts the porous carbon material precursor with the acid oralkali in a vapor phase can be mentioned. More specifically, whenconducting the treatment with the acid, the acid can be, for example, afluorine compound that shows acidity, such as hydrogen fluoride,hydrofluoric acid, ammonium fluoride, calcium fluoride or sodiumfluoride. When such a fluorine compound is used, the fluorine element isneeded to be in an amount 4 times as much as the silicon element in thesilicon component contained in the porous carbon material precursor, andthe concentration of an aqueous solution of the fluorine compound maypreferably be 10 wt % or higher. When eliminating, with hydrofluoricacid, the silicon component (for example, silicon dioxide) contained inthe porous carbon material precursor, silicon dioxide reacts withhydrofluoric acid as shown by the chemical equation (1) or chemicalequation (2), and is eliminated as hexafluorosilicic acid (H₂SiF₆) orsilicon tetrafluoride (SiF₄) so that a porous carbon material can beobtained. It is then necessary to conduct washing and drying.SiO₂+6HF→H₂SiF₆+2H₂O  (1)SiO₂+4HF→SiF₄+2H₂O  (2)

When conducting the treatment with the alkali (base), the alkali can be,for example, sodium hydroxide. When an aqueous solution of the alkali isused, the pH of the aqueous solution is needed to be 11 or higher. Wheneliminating, with an aqueous solution of sodium hydroxide, the siliconcomponent (for example, silicon dioxide) contained in the porous carbonmaterial precursor, heating of the aqueous solution of sodium hydroxidecauses silicon dioxide to react as indicated by the chemical equation(3) so that silicon dioxide is eliminated as sodium silicate (Na₂SiO₃)to obtain a porous carbon material. When sodium hydroxide is reacted ina vapor phase to conduct the treatment, on the other hand, heating ofsolid sodium hydroxide induces a reaction as indicated by the chemicalequation (3) so that silicon dioxide is eliminated as sodium silicate(Na₂SiO₃) to obtain a porous carbon material. It is then necessary toconduct washing and drying.SiO₂+2NaOH→Na₂SiO₃+H₂O  (3)

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. (A) and (B) of FIG. 1 are graphs presenting pore distributions ofmesopores and pore distributions of micropores in porous carbonmaterials of Example 1 and Comparative Example 1, respectively.

FIGS. (A) and (B) of FIG. 2 are graphs presenting pore distributions ofmesopores and pore distributions of micropores in porous carbonmaterials of Example 2 and Comparative Example 2, respectively.

FIGS. (A) and (B) of FIG. 3 are graphs presenting pore distributions ofmesopores and pore distributions of micropores in porous carbonmaterials of Example 3 and Comparative Example 3, respectively.

FIGS. (A) and (B) of FIG. 4 are graphs presenting a pore distribution ofmesopores and a pore distribution of micropores in a porous carbonmaterial of Example 4, respectively.

FIG. 5 is a graph showing the results obtained by measuring the pores invarious porous carbon materials by the mercury penetration method.

FIG. 6 is a graph showing the results of X-ray diffraction measurementsof various porous carbon materials.

FIG. 7 is a graph for describing a method that determines an R valuebased on the results of an X-ray diffraction measurement.

FIG. 8 is a schematic cross-sectional view of a lithium ion secondarybattery making use of a porous carbon material according to the presentinvention.

FIG. 9 is an enlarged view of a portion of a rolled electrode stack inthe lithium ion secondary battery depicted in FIG. 8.

FIG. 10 is a graph showing measurement results of the concentration ofibuprofen at respective times in Example 7.

FIGS. (A) and (B) of FIG. 11 are a schematic of an anti-pollinosis maskin Example 8 and a diagram illustrating a schematic cross-sectionalstructure of a main part of the anti-pollinosis mask, respectively.

DETAILED DESCRIPTION Best Modes for Carrying Out the Invention

With reference to the drawings, the present invention will hereinafterbe described based on Examples.

Example 1

Example 1 relates to a porous carbon material according to the presentinvention. In Example 1, rice (rice plant) husk was used as aplant-derived material which is a raw material for a porous carbonmaterial. The porous carbon material of Example 1 was obtained bycarbonizing the rice husk as a raw material into a carbonaceous material(porous carbon material precursor) and then applying an acid treatment.

In the production of the porous carbon material of Example 1, a heatingtreatment (precarbonizing treatment) was first applied to the rice husk(grown in Kagoshima Prefecture, husk of the Isehikari variety), whichhad been ground, in an inert gas. Specifically, the rice husk was heatedand charred at 500° C. for 5 hours in a nitrogen gas stream to obtain acharred material. It is to be noted that by conducting such a treatment,tar components which would be formed in the subsequent carbonization canbe decreased or eliminated. Subsequently, the charred material (10 g)was placed in an alumina-made crucible, and was heated to 1,000° C. at aramp-up rate of 5° C./min in a nitrogen gas stream (10 L/min). Thecharred material was then carbonized at 1,000° C. for 5 hours into acarbonaceous material (porous carbon material precursor), which wasthereafter allowed to cool down to room temperature. It is to be notedthat during the carbonization and cooling, nitrogen gas was caused toflow continuously. The porous carbon material precursor was thenimmersed overnight in a 46 vol % aqueous solution of hydrofluoric acidto conduct its acid treatment, followed by washing with water and ethylalcohol until pH 7 was reached. By finally conducting drying, it waspossible to obtain the porous carbon material of Example 1.

Using the same raw material as in Example 1, a porous carbon materialwas obtained as Comparative Example 1 based on a similar procedure as inExample 1 except that the acid treatment was not conducted.

With respect to the porous carbon materials of Example 1 and ComparativeExample 1, their specific surface areas and pore volumes were measured.The results shown in Table 1 were obtained. With respect to the porouscarbon materials of Example 1 and Comparative Example 1, the pore sizedistributions of their mesopores and micropores were measured. Theresults shown in FIGS. (A) and (B) of FIG. 1 were obtained.

Nitrogen adsorption and desorption tests were conducted usingBELSORP-mini (manufactured by BEL Japan Inc.) as a measuring instrumentfor the determination of the specific surface areas and pore volumes. Asa measuring condition, the measurement equilibrium relative pressure(p/p₀) was set at 0.01 to 0.95. Based on the BELSORP Analysis Software,the specific surface areas and pore volumes were calculated. Further,nitrogen adsorption and desorption tests were conducted using theabove-mentioned measuring instrument, and the pore size distributions ofthe mesopores and micropores were calculated by the BELSORP AnalysisSoftware on the basis of the BJH method and MP method. It is to be notedthat in Examples, Comparative Examples and Referential Examples to bedescribed subsequently herein, the measurements of specific surfacesareas and pore volumes and pore size distributions of mesopores andmicropores were conducted by similar methods.

As shown in Table 1, the specific surface area and pore volume of theporous carbon material of Example 1 in which the acid treatment wasconducted were considerably large compared with the specific surfacearea and pore volume of the porous carbon material of ComparativeExample 1 in which no acid treatment was conducted, and the value ofspecific surface area was 400 m²/g or greater and the value of porevolume was 0.1 cm³/g or greater. It was also found that as shown in FIG.(A) of FIG. 1, the porous carbon material of Example 1 contained manymesopores of 20 nm and smaller in pore size, especially many mesoporesof 10 nm and smaller in pore size compared with the porous carbonmaterial of Comparative Example 1. It was also found that as shown inFIG. (B) of FIG. 1, the porous carbon material of Example 1 containedmany micropores of approx. 1.9 nm in pore size, many micropores ofapprox. 1.5 nm in pore size and many micropores of approx. 0.8 nm to 1nm in pore size compared with the porous carbon material of ComparativeExample 1.

The porous carbon materials of Example 1 and Comparative Example 1 werealso subjected to an elemental analysis, and the results shown in Table2 were obtained. It is to be noted that using an energy dispersive X-rayanalyzer (JED-2200F manufactured by JEOL Ltd. (trademark)) as ameasuring instrument for elemental analysis, each element wasquantitated by energy dispersion spectroscopy (EDS) and its content wasthen calculated in terms of percentage by weight (wt %). As measurementconditions, the scanning voltage and irradiation current were set at 15kV and 13 μA, respectively. They were set likewise in the subsequentExamples and Comparative Examples.

As shown in Table 2, the porous carbon material of Example 1 in whichthe acid treatment was conducted was lower in the contents of silicon(Si), oxygen (O), potassium (K), calcium (Ca) and sodium (Na) than theporous carbon material of Comparative Example 1 in which no acidtreatment was conducted. In particular, the contents of silicon (Si) andoxygen (O) substantially decreased in Example 1 than in ComparativeExample 1, and became 1 wt % or lower. On the other hand, the contentsof phosphorus (P) and sulfur (S) increased more in Example 1 than inComparative Example 1. From the foregoing, it has been confirmed that ina porous carbon material produced by carbonizing rice husk as a rawmaterial at 800° C. to 1,400° C. and then conducting a treatment with anacid, the content of silicon (Si) becomes 1 wt % or lower, the contentof magnesium (Mg) becomes at least 0.01 wt % but at most 3 wt %, thecontent of potassium (K) becomes at least 0.01 wt % but at most 3 wt %,and the content of calcium (Ca) becomes at least 0.05 wt % but at most 3wt %. It has also been confirmed that the content of phosphorus (P)becomes at least 0.01 wt % but at most 3 wt % and the content of sulfur(S) becomes at least 0.01 wt % but at most 3 wt %. It is to be notedthat as other elements, although the types of elements are not shown,carbon (C) was most abundant and carbon (C) amounted to 90% or more ofthe other elements. Here, silicon is contained as an amorphous silicacomponent in rice husk, and the content of silicon in the rice husk asthe raw material was 9.4 wt %.

As the porous carbon material of Example 1 substantially decreased inthe contents of silicon (Si) and oxygen (O) than the porous carbonmaterial of Comparative Example 1, it was also suggested from theanalysis results of Example 1 that silicon dioxide was abundantlycontained in the carbonaceous material (porous carbon materialprecursor). It is, accordingly, suggested that the treatment of a porouscarbon material precursor with an acid eliminates contained siliconcomponents such as silicon dioxide and contributes to an increase in thevalue of specific surface area. Further, it has been confirmed that bythe treatment with an acid, mesopores and micropores increase. Similarsuggestions and confirmation were derived from the Examples to bedescribed subsequently herein. Similar results were also obtained with aporous carbon material obtained by conducting a treatment with an alkali(base) such as an aqueous solution of sodium hydroxide as an alternativeto aqueous solution of hydrofluoric acid.

Example 2

Example 2 is a modification of Example 1. In Example 2, straw of riceplant (grown in Kagoshima Prefecture; the Isehikari variety) was used asa plant-derived material which was a raw material for a porous carbonmaterial. The porous carbon material of Example 2 was obtained bycarbonizing the straw as a raw material into a carbonaceous material(porous carbon material precursor) and then applying an acid treatment.It is to be noted that a similar process as in Example 1 was employedfor the production of the porous carbon material. Using the same rawmaterial as in Example 2, a porous carbon material was obtained asComparative Example 2 based on a similar procedure as in Example 1except that the acid treatment was not conducted.

With respect to the porous carbon materials of Example 2 and ComparativeExample 2, their specific surface areas and pore volumes were measured.The results shown in Table 1 were obtained. The pore size distributionsof their mesopores and micropores were also measured. The results shownin FIGS. (A) and (B) of FIG. 2 were obtained.

As shown in Table 1, it was found that the specific surface area andpore volume of the porous carbon material of Example 2 in which the acidtreatment was conducted were considerably large compared with thespecific surface area and pore volume of the porous carbon material ofComparative Example 2 in which no acid treatment was conducted and thatthe value of specific surface area was 100 m²/g or greater and the valueof pore volume was 0.1 cm³/g or greater. It was also found that as shownin FIG. (A) of FIG. 2, the porous carbon material of Example 2 containedmany mesopores of 20 nm and smaller in pore size, especially manymesopores of 10 nm and smaller in pore size compared with the porouscarbon material of Comparative Example 2. It was also found that asshown in FIG. (B) of FIG. 2, the porous carbon material of Example 2contained many micropores of approx. 1.9 nm in pore size, manymicropores of approx. 1.5 nm in pore size and many micropores of approx.0.8 nm to 1 nm in pore size compared with the porous carbon material ofComparative Example 2.

The porous carbon materials of Example 2 and Comparative Example 2 werealso subjected to an elemental analysis, and the results shown in Table2 were obtained.

As shown in Table 2, the porous carbon material of Example 2 in whichthe acid treatment was conducted was lower in the contents of silicon(Si), oxygen (O), magnesium (Mg), potassium (K) and sodium (Na) than theporous carbon material of Comparative Example 2 in which no acidtreatment was conducted. In particular, the contents of silicon (Si) andoxygen (O) substantially decreased in Example 2 than in ComparativeExample 2, and became 1 wt % or lower. On the other hand, the contentsof phosphorus (P), sulfur (S) and calcium (Ca) increased more in Example2 than in Comparative Example 2. From the foregoing, it has beenconfirmed that in a porous carbon material produced by carbonizing strawas a raw material at 800° C. to 1,400° C. and then conducting atreatment with an acid, the content of silicon (Si) also becomes 1 wt %or lower, the content of magnesium (Mg) also becomes at least 0.01 wt %but at most 3 wt %, the content of potassium (K) also becomes at least0.01 wt % but at most 3 wt %, and the content of calcium (Ca) alsobecomes at least 0.05 wt % but at most 3 wt %. It has also beenconfirmed that the content of phosphorus (P) becomes at least 0.01 wt %but at most 3 wt % and the content of sulfur (S) becomes at least 0.01wt % but at most 3 wt %. It is to be noted that as other elements,although the types of elements are not shown, carbon (C) was mostabundant and carbon (C) amounted to 90% or more of the other elements.Here, silicon is contained as an amorphous silica component in straw,and the content of silicon in the straw as the raw material was 6.8 wt%.

Example 3

Example 3 is also a modification of Example 1. In Example 3, grass reed(cut in December, 2006 in Aoba Ward, Yokohama City; withered in winter)was used as a plant-derived material which was a raw material for aporous carbon material. The porous carbon material of Example 3 wasobtained by carbonizing the grass reed as a raw material into acarbonaceous material (porous carbon material precursor) and thenapplying an acid treatment. It is to be noted that a similar process asin Example 1 was employed for the production of the porous carbonmaterial. Using the same raw material as in Example 3, a porous carbonmaterial was obtained as Comparative Example 3 based on a similarprocedure as in Example 1 except that the acid treatment was notconducted.

With respect to the porous carbon materials of Example 3 and ComparativeExample 3, their specific surface areas and pore volumes were measured.The results shown in Table 1 were obtained. The pore size distributionsof their mesopores and micropores were also measured. The results shownin FIGS. (A) and (B) of FIG. 3 were obtained.

As shown in Table 1, it was found that the specific surface area andpore volume of the porous carbon material of Example 3 in which the acidtreatment was conducted were considerably large compared with thespecific surface area and pore volume of the porous carbon material ofComparative Example 3 in which no acid treatment was conducted and thatthe value of specific surface area was 100 m²/g or greater and the valueof pore volume was 0.1 cm³/g or greater. It was also found that as shownin FIG. (A) of FIG. 3, the porous carbon material of Example 3 containedmany mesopores of 20 nm and smaller in pore size, especially manymesopores of 10 nm and smaller in pore size compared with the porouscarbon material of Comparative Example 3. It was also found that asshown in FIG. (B) of FIG. 3, the porous carbon material of Example 3contained many micropores of approx. 1.9 nm in pore size, manymicropores of approx. 1.5 nm in pore size, many micropores of approx.1.3 nm in pore size and many micropores of approx. 0.8 nm to 1 nm inpore size compared with the porous carbon material of ComparativeExample 3.

The porous carbon materials of Example 3 and Comparative Example 3 werealso subjected to an elemental analysis, and the results shown in Table2 were obtained.

As shown in Table 2, the porous carbon material of Example 3 in whichthe acid treatment was conducted was lower in the contents of silicon(Si) and oxygen (O) than the porous carbon material of ComparativeExample 3 in which no acid treatment was conducted. In particular, thecontents of silicon (Si) and oxygen (O) substantially decreased inExample 3 than in Comparative Example 3, and became 1 wt % or lower. Onthe other hand, the contents of phosphorus (P), sulfur (S), potassium(K) and calcium (Ca) increased more in Example 3 than in ComparativeExample 3. From the foregoing, it has been confirmed that in a porouscarbon material produced by carbonizing reed as a raw material at 800°C. to 1,400° C. and then conducting a treatment with an acid, thecontent of silicon (Si) also becomes 1 wt % or lower, the content ofmagnesium (Mg) also becomes 0.01 wt % or higher but 3 wt % or lower, thecontent of potassium (K) also becomes at least 0.01 wt % but at most 3wt %, and the content of calcium (Ca) also becomes at least 0.05 wt %but at most 3 wt %. It has also been confirmed that the content ofphosphorus (P) becomes at least 0.01 wt % but at most 3 wt % and thecontent of sulfur (S) becomes at least 0.01 wt % but at most 3 wt %. Itis to be noted that as other elements, although the types of elementsare not shown, carbon (C) was most abundant and carbon (C) amounted to90% or more of the other elements. Here, silicon is contained as anamorphous silica component in reed, and the content of silicon in thereed as the raw material was 4.8 wt %.

Example 4

Example 4 is also a modification of Example 1. In Example 4, seaweedstem (grown in the Sanriku district of Iwate Prefecture) was used as aplant-derived material which was a raw material for a porous carbonmaterial. The porous carbon material of Example 4 was obtained bycarbonizing the seaweed stem as a raw material into a carbonaceousmaterial (porous carbon material precursor) and then applying an acidtreatment.

Specifically, the seaweed stem was first heated, for example, at atemperature of 500° C. or so to char the same. It is to be noted thatbefore the heating, the seaweed stem as a raw material may be treatedwith an alcohol, for example. As a specific treatment method, the methodthat immerses the seaweed stem in ethyl alcohol or the like can bementioned. By this treatment, the water contained in the raw materialcan be decreased, and at the same time, non-carbon elements and mineralcomponents which would be contained in a porous carbon material to beobtained finally can be dissolved out. By this alcohol treatment, it isalso possible to reduce the occurrence of gas during carbonization. Morespecifically, the seaweed stem was immersed for 48 hours in ethylalcohol. It is to be noted that ultrasonication was applied in ethylalcohol. Next, the seaweed stem was heated and charred at 500° C. for 5hours in a nitrogen gas stream to obtain a charred material. It is to benoted that by conducting such a treatment (precarbonization treatment),tar components which would be formed in the subsequent carbonization canbe decreased or eliminated. Subsequently, the charred material (10 g)was placed in an alumina-made crucible, and was heated to 1,000° C. at aramp-up rate of 5° C./rain in a nitrogen gas stream (10 L/min). Thecharred material was then carbonized at 1,000° C. for 5 hours into acarbonaceous material (porous carbon material precursor), which wasthereafter allowed to cool down to room temperature. It is to be notedthat during the carbonization and cooling, nitrogen gas was caused toflow continuously. The porous carbon material precursor was thenimmersed overnight in a 46 vol % aqueous solution of hydrofluoric acidto conduct its acid treatment, followed by washing with water and ethylalcohol until pH 7 was reached. By finally conducting drying, it waspossible to obtain the porous carbon material of Example 4.

With respect to the porous carbon material of Example 4, its specificsurface area and pore volume were measured. The results shown in Table 1were obtained. The pore size distributions of its mesopores andmicropores were also measured. The results shown in FIGS. (A) and (B) ofFIG. 4 were obtained.

As shown in Table 1, it was found that the values of specific surfacearea and pore volume of the porous carbon material of Example 4 became400 m²/g or greater and 0.1 cm³/g or greater, respectively. It was foundthat as shown in FIG. (A) of FIG. 4, the porous carbon material ofExample 4 contained many mesopores of 20 nm to 25 nm in pore size andmany mesopores of 15 nm and smaller in pore size. It was also found thatas shown in FIG. (B) of FIG. 4, the porous carbon material of Example 4contained many micropores of approx. 1.8 nm to 2.0 nm in pore size, manymicropores of approx. 1.4 nm to 1.6 nm in pore size, and many microporesof approx. 0.5 nm to 1 nm in pore size. An elemental analysis was alsoconducted on the porous carbon material of Example 4, and the resultsshown in Table 2 were obtained.

As has been described above, it has been confirmed from the results ofTable 1 and Table 2 that by conducting carbonization at 800° C. to1,400° C. and then a treatment with an acid, the resulting porous carbonmaterial, irrespective of the kind of a plant as a raw material, has avalue of specific surface area of 10 m²/g or greater as measured by thenitrogen BET method and a silicon (Si) content of 1 wt % or lower.

TABLE 1 Specific surface area Pore volume Example Comp. Ex. (m²/g)(cm³/g) 1 429 0.47 1 6.26 0.018 2 130 0.14 2 2.46 0.014 3 141.9 0.12 31.57 0.0046 4 492 0.34

TABLE 2 Ex. Comp. Ex. O Na Mg Si P S K Ca Others 1 <0.01 <0.01 0.09 0.910.23 0.07 0.18 0.15 98.37 1 22.77 0.05 0.08 19.10 0.03 0.02 0.59 0.2057.16 2 <0.01 <0.01 0.17 0.97 0.39 0.20 0.71 0.55 97.01 2 15.20 0.170.24 13.84 0.17 0.13 1.77 0.42 68.06 3 <0.01 <0.01 0.10 0.71 0.41 0.170.82 0.42 97.37 3 8.73 <0.01 0.02 9.71 0.13 0.11 0.68 0.1 80.88 4 <0.01<0.01 1.99 <0.01 0.48 2.70 <0.01 6.74 Ex. V Cu Z Se Others 4 <0.01 0.210.24 <0.01 87.91

For reference, pores in various porous carbon materials were measured bythe mercury penetration method. Specifically, using a mercuryporosimeter (PASCAL440; manufactured by Thermo Electric Corporation),measurements were conducted by the mercury penetration method. Themeasurement range of pores was set at 10 μm to 2 nm. The results areshown in FIG. 5. The samples of the various porous carbon materials areas shown below in Table 3. It was confirmed that the pore volumes of theporous carbon materials according to the present invention as determinedby the mercury penetration method were significantly increased by theapplication of an acid treatment with an aqueous solution ofhydrofluoric acid (expressed as “hydrofluoric acid treatment” in thetable). In addition, it was found that the pore volumes were greaterthan those of commercially-available activated carbons (which areReferential Example 6-1 and Referential Example 6-2 and will bedescribed in detail in Example 6) and reached 2.2 cm³/g or more.

TABLE 3 Pore volume Sample Treatment Raw material (cm³/g) a Carbonizedat 800° C.; Rice husk 2.88 hydrofluoric acid treatment applied bCarbonized at 800° C.; 1.36 hydrofluoric acid treatment not applied cCarbonized at 1,000° C.; Rice husk 3.45 hydrofluoric acid treatmentapplied d Carbonized at 1,000° C.; 1.39 hydrofluoric acid treatment notapplied e Carbonized at 1,000° C.; Straw 2.94 hydrofluoric acidtreatment applied f Carbonized at 1,000° C.; 2.14 hydrofluoric acidtreatment not applied g Carbonized at 1,000° C.; Reed 2.24 hydrofluoricacid treatment applied h Carbonized at 1,000° C.; 1.84 hydrofluoric acidtreatment applied i Referential Example 6-1 1.50 j Referential Example6-2 1.77

Further, the results of evaluations of the various porous carbonmaterials by powder X-ray diffractometry are shown in FIG. 6. Here, anX-ray diffractometer (RINT-TTRII) manufactured by RIGAKU Corporation wasused, and Cu—Kα radiation was employed as an X-ray source. It is to benoted that the wavelength was 0.15405 nm. In addition, the appliedvoltage was set at 50 kilovolts, and the scanning step was set at 0.04°.As a result of the analysis by powder X-ray diffractometry, the porouscarbon materials according to the present invention (the sample a,sample b, sample e, sample f, sample g, and sample h shown in Table 3)were confirmed to be higher in crystallinity than thecommercially-available activated carbon (Referential Example 6-2) fromthe intensities of diffraction peaks around 25° diffraction angle 2□[diffraction peaks of (002) planes].

With reference to a technical paper, Weibing Xing, J. S. Xue, Tao Zheng,A. Gibaud and J. R. Dahn, J. Electrochem. Soc. Vol. 143, 3482 (1996),the calculation of an R value which is an empirical parametercorrelating to the number of graphene sheets was conducted.Specifically, as illustrated in FIG. 7, the R value (=B/A) wasdetermined by conducting fitting, and the R value was considered tocorrelate to the number of graphene sheets in a porous carbon material.Namely, the greater the R value, was considered the higher (better) thecrystallinity of the porous carbon material. It is to be noted that FIG.7 diagrammatically illustrates a method that determines an R value byusing the results of powder X-ray diffraction (XRD) of the sample ashown in Table 3. More specifically, when the intensity (count) at anintersection between the baseline BL of a diffraction peak of the (002)plane as obtained based on powdery X-ray diffractometry of the porouscarbon material and a perpendicular line NL downwardly drawn from thediffraction peak of the (002) plane is assumed to be an “A value” andthe intensity (count) of the diffraction peak of the (002) plane isassumed to be a “B value”, the R value can be expressed as R=B/A. FromTable 4, it has been found that in a porous carbon material according tothe present invention, the R value is, for example, 1.5 or greater, morespecifically, 1.8 or greater.

TABLE 4 Raw A B R Treatment material value value value Carbonized at800° C.; Rice husk 6.5 11.6 1.78 hydrofluoric acid treatment appliedCarbonized at 1,200° C.; Rice husk 6.2 12.8 2.06 hydrofluoric acidtreatment applied Carbonized at 1,000° C.; Straw 5.3 13.0 2.45hydrofluoric acid treatment applied Carbonized at 1,000° C.; Reed 5.212.7 2.44 hydrofluoric acid treatment applied Referential Example 6-24.3 5.5 1.28

Example 5

In Example 5 to Example 8, a description will next be made aboutapplication examples of the porous carbon materials described in Example1 to Example 4. In Example 5, lithium ion secondary batteries werefabricated as electrochemical devices. Anode active material layers wereformed with the porous carbon materials described in Example 1 toExample 4, respectively. A schematic cross-sectional view of eachlithium ion secondary battery is depicted in FIG. 8, and an enlargedview of a portion of a rolled electrode stack depicted in FIG. 8 isillustrated in FIG. 9.

In the lithium ion secondary battery, the capacity of an anode isexpressed by a capacity component based on occlusion and release oflithium as an electrode reactant, and the lithium ion secondary batteryhas a battery structure called the so-called cylindrical type.Specifically, in the lithium ion secondary battery, a rolled electrodestack 20 with a cathode 21 and an anode 22 rolled with a separator 23interposed therebetween and a pair of insulating plates 12, 13 areaccommodated inside a substantially hollow cylindrical battery can 11.The battery can 11 is made, for example, of iron to which nickel platinghas been applied, is closed at an end portion thereof, and is open at anopposite end portion thereof to form an open end portion. The pairedinsulating plates 12, 13 are arranged such that they hold the rolledelectrode stack 20 therebetween and are located at right angles to arolled peripheral wall thereof.

In the open end portion of the battery can 11, a battery cover 14, and asafety valve mechanism 15 and a positive temperature coefficient (PTC)device 16, which are arranged inside the battery cover 14, are assembledby staking them together via a gasket 17, so that the interior of thebattery can 11 is sealed. The battery cover 14 is made of a similarmaterial as the battery can 11, for example. The safety valve mechanism15 is electrically connected to the battery cover 14 by way of thepositive temperature coefficient device 16. The safety valve mechanism15 is constructed such that, if the internal pressure rises to apredetermined level or higher due to internal short-circuiting, heatingfrom the outside, or the like, a disk plate 15A is bulged out to cut offthe electrical connection between the battery cover 14 and the rolledelectrode stack 20. Responsive to a rise in temperature, the positivetemperature coefficient device 16 increases in resistance to limit acurrent, thereby preventing abnormal heat evolution which wouldotherwise take place due to a large current. The gasket 17 is made, forexample, of an insulating material, and is coated at its surfaces withasphalt.

For example, a center pin 24 is centrally inserted in the rolledelectrode stack 20. In this rolled electrode stack 20, a cathode lead 25made of aluminum or the like is connected to the cathode 21, while ananode lead 26 made of nickel or the like is connected to the anode 22.The cathode lead 25 is welded to the safety valve mechanism 15, andtherefore, is electrically connected to the battery cover 14. On theother hand, the anode lead 26 is welded to the battery can 11.

In the cathode 21, cathode active material layers 21B are arranged, forexample, on both sides of a cathode current collector 21A having theopposite sides in pair. It is to be noted that a cathode active materiallayer 21B may be arranged on only one side of the cathode currentcollector 21A. The cathode current collector 21A is made of a metalmaterial such as, for example, aluminum, nickel or stainless steel. Forexample, the cathode active material layers 21B contain, as a cathodeactive material, any one, two or more of cathode materials capable ofoccluding and releasing lithium as the electrode reactant. The cathodeactive material layers 21B may contain a conductive agent, a binderand/or the like as needed.

As a cathode material capable of occluding and releasing lithium, alithium-containing compound can be mentioned, for example. With thelithium-containing compound, a high energy density can be obtained. Thelithium-containing compound can be, for example, a lithium compositeoxide containing lithium and a transition metal element, or a phosphatecompound containing lithium and a transition metal element. As thetransition metal element, at least one of cobalt, nickel, manganese andiron can be mentioned in particular. With such a lithium-containingcompound, a still higher voltage can be obtained. Its chemical formulacan be represented, for example, by Li_(x)M1O₂ or Li_(y)M2PO₄. In thesechemical formulas, M1 and M2 represent one or more transition metalelements. The values of x and y differ depending on the charge-dischargestate of the battery, but in general, 0.05≤x≤1.10 and 0.05≤y≤1.10.

The lithium composite oxide containing lithium and the transition metalelement can be, for example, a lithium-cobalt composite oxide(Li_(x)CoO₂), lithium-nickel composite oxide (Li_(x)NiO₂),lithium-nickel-cobalt composite oxide (Li_(x)Ni_((1−z))Co_(z)O₂(z<1)),or lithium-nickel-cobalt-manganese composite oxide(Li_(x)Ni_((1−v−w))Co_(v)Mn_(w)O₂(v+w<1)); a lithium-manganese compositeoxide (LiMn₂O₄) having the spinel structure; or the like. Among these, anickel-containing, lithium composite oxide is preferred. With such anickel-containing, lithium composite oxide, a high capacity can beobtained, and also, excellent cycling characteristics can be obtained.As the phosphate compound containing lithium and the transition metalelement, on the other hand, a lithium-iron-phosphate compound (LiFePO₄)or lithium-iron-manganese-phosphate compound(LiFe_((1−u))Mn_(u)PO₄(u<1)), for example, can be mentioned.

In addition, an oxide such as titanium oxide, vanadium oxide ormanganese dioxide, a disulfide such as iron disulfide, titaniumdisulfide or molybdenum disulfide, a chalcogenated compound such asniobium selenide, sulfur, or a conductive polymer such as polyaniline orpolythiophene can also be mentioned, for example.

In the anode 22, anode active material layers 22B are arranged, forexample, on both sides of an anode current collector 22A having theopposite sides in pair. It is to be noted that an anode active materiallayer 22B may be arranged on only one side of the anode currentcollector 22A. The anode current collector 22A is made of a metalmaterial such as, for example, copper (Cu), nickel or stainless steel.The anode active material layers 22B contain, as an anode activematerial, an anode material capable of occluding and releasing lithiumas the electrode reactant, and in addition, may contain a conductiveagent, a binder and/or the like as needed.

The anode active material layers 22B are composed of one of the porouscarbon materials described in Example 1 to Example 4. With the use ofone of the porous carbon materials described in Example 1 to Example 4,the changes in crystalline structure, which occur upon charging anddischarging, can be controlled to very little, thereby making itpossible to obtain a high energy density. Moreover, the acceptability oflithium is high and the deposition of lithium is inhibited, so that thereduction in discharge capacity can be suppressed. Owing to these,improvements can be achieved in cycling characteristics and storagecharacteristics. It is to be noted that the anode active material layers22B may also contain one or more anode materials, which can occlude andrelease lithium, together with one of the porous carbon materials ofExample 1 to Example 4.

As the conductive agent, a carbon material such as graphite, carbonblack or Ketjenblack can be mentioned, for example. They may be usedeither singly, or plural ones of them may be used in combination. As analternative, the conductive agent can also be any metal material insofaras it is a material having electrical conductivity, a conductivepolymer, or the like. As the binder, on the other hand, for example, asynthetic rubber such as a styrene-butadiene-based rubber, fluorinatedrubber or ethylene-propylene-diene or a polymer material such aspolyfluorinated vinylidene can be mentioned. They may be used eithersingly, or plural ones of them may be used in combination.

In the lithium ion secondary battery, adjustments of the amounts of thecathode active material and anode active material make the chargecapacity of the anode active material greater than the charge capacityof the cathode active material so that lithium metal does not deposit onthe anode 22 even at the time of full charge.

The separator 23 isolates the cathode 21 and the anode 22 from eachother, and allows lithium ions to pass therethrough while preventingshort-circuiting of a current due to a contact between both electrodes.The separator 23 is formed, for example, of a porous membrane made of asynthetic resin comprising polytetrafluoroethylene, polypropylene orpolyethylene or the like, or a porous membrane made of ceramics, or mayhave a structure that two or more of these porous membranes arelaminated. Among such porous membranes, a polyolefin-made, porousmembrane is preferred because it is excellent in short-circuitpreventing effect and can achieve an improvement in the safety of thelithium ion secondary battery owing to the shutdown effect. Inparticular, polyethylene is a preferred material because it can obtainthe shutdown effect in a temperature range of 100° C. or higher but 160°C. or lower and is excellent in electrochemical stability. In addition,polypropylene is also preferred. Further, a resin copolymerized withpolyethylene or polypropylene or a resin obtained by blendingpolyethylene or polypropylene can also be used insofar as the resin isprovided with chemical stability. The separator 23 is impregnated withan electrolyte solution as a liquid electrolyte. In the electrolytesolution, a solvent and an electrolyte salt dissolved in the solvent arecontained.

The solvent contains, for example, a nonaqueous solvent such as anorganic solvent. Examples of the nonaqueous solvent can include ethylenecarbonate, propylene carbonate, butylene carbonate, dimethyl carbonate,diethyl carbonate, ethyl methyl carbonate, methyl propyl carbonate,γ-butyrolactone, γ-valerolactone, 1,2-dimethoxyethane, tetrahydrofuran,2-methyltetrahydrofuran, tetrahydropyran, 1,3-dioxolane,4-methyl-1,3-dioxolane, 1,3-dioxane, 1,4-dioxane, methyl acetate, ethylacetate, methyl propionate, ethyl propionate, methyl butyrate, methylisobutyrate, methyl trimethylacetate, ethyl trimethylacetate,acetonitrile, glutaronitrile, adiponitrile, methoxyacetonitrile,3-methoxypropionitrile, N,N-dimethylformamide, N-methylpyrrolidinone,N-methyloxazolidinone, N,N′-dimethylimidazolidinone, nitromethane,nitroethane, sulfolane, trimethyl phosphate, dimethyl sulfoxide, anddimethyl sulfoxide phosphate. These nonaqueous solvents may be usedeither singly, or plural ones of them may be used in combination. Thesolvent may preferably contain at least one nonaqueous solvent selectedfrom the group consisting of ethylene carbonate, propylene carbonate,dimethyl carbonate, diethyl carbonate and ethyl methyl carbonate. Withsuch a solvent, sufficient cycling characteristics are obtained. In thiscase, it is particularly preferred to use a mixture of a high-viscosity(high permittivity) solvent (for example, the relative permittivity ε ofwhich is 30 or higher) such as ethylene carbonate or propylene carbonateand a low-viscosity solvent (for example, the viscosity of which is 1mPa·s or lower) such as dimethyl carbonate, ethyl methyl carbonate ordiethyl carbonate. With such a mixture, the dissociation property of anelectrolyte salt and the ionic mobilities are improved so that stillhigher effects can be obtained. It is to be noted that in addition tothe above-described nonaqueous solvent, vinylene carbonate,fluoroethylene carbonate and/or the like may also be contained in thesolvent.

The electrolyte salt contains, for example, one or more light metalsalts such as lithium salts. The lithium salts can be, for example,lithium hexafluorophosphate (LiPF₆), lithium tetrafluoroborate (LiBF₄),lithium perchlorate (LiClO₄), lithium hexafluoroarsenate (LiAsF₆),lithium tetraphenylborate (LiB(C₆H₅)₄), lithium methanesulfonate(LiCH₃SO₃), lithium trifluoromethanesulfonate (LiCF₃SO₃), lithiumtetrachloroaluminate (LiAlCl₄), dilithium hexafluorosilicate (Li₂SiF₆),lithium chloride (LiCl), and lithium bromide (LiBr). They may be usedsingly, or plural ones of them may be used in combination.

The content of the electrolyte salt may preferably be in a range of atleast 0.3 mol/kg but at most 3.0 mol/kg based on the solvent. Outsidethis range, the ionic conductivity is extremely lowered, leading to apotential problem that the capacity characteristic and the like may notbe obtained fully.

The lithium ion secondary battery can be fabricated, for example, asfollows.

For example, the cathode active material layers 21B are first formed onboth sides of the cathode current collector 21A to prepare the cathode21. Upon formation of cathode active material layers 21B, a cathode mixwhich is a mixture of powder of a cathode active material, a conductiveagent and a binder is dispersed in a solvent to prepare a paste-likeslurry of the cathode mix. After the slurry of the cathode mix is coatedon the cathode current collector 21A and is then dried, compressionforming is performed. Following a similar procedure as that for thecathode 21, for example, the anode active material layers 22B are alsoformed on both sides of the anode current collector 22A to prepare theanode 22. Specifically, upon formation of the anode active materiallayers 22B, an anode mix which is a mixture of one of the porous carbonmaterials described in Example 1 to Example 4, a conductive agent and abinder is dispersed in a solvent to prepare a paste-like slurry of theanode mix. After the slurry of the anode mix is coated on the anodecurrent collector 22A and is then dried, compression forming isperformed.

Next, the cathode lead 25 is welded to the cathode current collector21A, and the anode lead 26 is welded to the anode current collector 22A.Subsequently, the cathode 21 and anode 22 are rolled with the separator23 interposed therebetween to form the rolled electrode stack 20. Afterthe cathode lead 25 is welded at an end portion thereof to the safetyvalve mechanism 15 and the anode lead 26 is welded at an end portionthereof to the battery can 11, the rolled electrode stack 20 isaccommodated inside the battery can 11 while holding it between thepaired insulating plates 12, 13. An electrolyte solution is then chargedinto the battery can 11 to have the separator 23 impregnated with theelectrolyte solution. Finally, the battery cover 14, safety valvemechanism 15 and positive temperature coefficient device 16 are fixed inthe open end portion of the battery can 11 by staking them by way of thegasket 17. In this manner, the lithium ion secondary battery depicted inFIG. 8 and FIG. 9 can be completed.

In the lithium ion secondary battery, when charging is performed,lithium ions are released, for example, from the cathode 21 and areoccluded in the anode 22 via the electrolyte solution. When dischargingis performed, on the other hand, lithium ions are released, for example,from the anode 22 and are occluded in the cathode 21 via the electrolytesolution.

As the anode active material is composed of one of the porous carbonmaterials of Example 1 to Example 4 in the lithium ion secondarybattery, excellent characteristics are obtained.

Example 6

Example 6 relates to the adsorbent according to the present invention.In Example 6, a porous carbon material [a plant-derived material as araw material for the porous carbon material was the same rice husk as inExample 1 (grown in Kagoshima Prefecture, rice husk of Isehikarivariety)] was applied as a porous carbon material for selectivelyadsorbing various unnecessary molecules in the body. With respect tovarious substances, their adsorbed amounts per unit weight of the porouscarbon material were measured.

Upon measurement of the adsorbed amounts, using four substances ofdifferent number average molecular weights, creatinine (number averagemolecular weight: 131), alizarin cyanine green (number average molecularweight: 623), lysozyme (number average molecular weight: 14,307),albumin (number average molecular weight: approx. 66,000), and aphosphate buffer of pH 7.3, solutions (aqueous solution A, aqueoussolution B, aqueous solution C, aqueous solution D) of theconcentrations shown below in Table 5 were prepared. It is to be notedthat the concentrations of the respective aqueous solutions beforeadsorption were determined as desired. To each of aliquots (40.0 mL) ofthe thus-prepared solutions, the porous carbon material (0.010 g) wasadded, followed by shaking at 37±2° C. for 1 hour. Subsequent to theshaking, the porous carbon material was removed from the solution byusing a polytetrafluoroethylene-made membrane filter having 500-μmpores. The absorbance of the filtrate was measured by a UV-visibleabsorbance measurement to determine the molar concentration of theaqueous solution. By a comparison with the molar concentration of theinitial aqueous solution before the adsorption, the adsorbed amount wascalculated. The adsorbed amount per gram of the porous carbon materialwas calculated based on the following formula.(Adsorbed amount per gram of porous carbon material)=(molecular weightof solute)×{(molar concentration of aqueous solution beforeadsorption)−(molar concentration of aqueous solution afteradsorption)}/(amount of porous carbon material per 1,000 mL)

In Example 6, the porous carbon materials shown below in Table 6 wereproduced. It is to be noted that Example 6-1 in Table 6 is a porouscarbon material produced by the same procedure as in Example 1 (exceptthat the carbonization temperature and the carbonization time were setat 800° C. and 1 hour, respectively) and that in Example 6-2, Example6-3, Example 6-4 and Example 6-5, the activation treatments shown inTable 6 were applied to the porous carbon material of Example 6-1,respectively. It is also to be noted that in Example 6-2, amicrostructure was allowed to develop with volatile components andcarbon molecules in the porous carbon material by using oxygen as anactivator and heating the porous carbon material at 900° C. for 2 hoursin air. In Example 6-3 to Example 6-5, on the other hand, microstructurewere allowed to develop with volatile components and carbon molecules inthe porous carbon material by using steam as an activator and heatingthe porous carbon material at 900° C. for 30 minutes, 1 hour and 2hours, respectively, under a steam atmosphere. The measurement resultsof specific surface area and the measurement results of pore volume arealso shown in Table 6. It is appreciated from Table 6 that in Example6-3 to Example 6-5, the value of specific surface area and the value ofpore volume increased with the time of the activation treatment.

For reference, using the activated carbons shown below in Table 7,adsorbed amounts per gram were also measured as Referential Example 6-1,Referential Example 6-2, Referential Example 6-3 and Referential Example6-4.

TABLE 5 Molecular Molar weight of concentration Solute solute (mol/L)Aq. soln. A Creatinine 131 3.567 × 10⁻⁴ Aq. soln. B Alizarin 623 7.774 ×10⁻⁵ cyanine green Aq. soln. C Lysozyme 14307 8.370 × 10⁻⁵ Aq. soln. DAlbumin 66000 6.533 × 10⁻⁵

TABLE 6 Specific Pore Treatment after acid surface area volume Exampletreatment (m²/g) (cm³/g) 6-1 None 589 0.60 6-2 Air-activated 951 1.68900° C. × 2 hours 6-3 Steam-activated 727 0.63 900° C. × 30 minutes 6-4Steam-activated 836 0.66 900° C. × 1 hour 6-5 Steam-activated 930 0.80900° C. × 2 hours

TABLE 7 Specific surface Pore Product Raw area volume Ref. Ex. nameManufacturer material (m²/g) (cm³/g) 6-1 Activated Wako Pure Petroleum1231 0.57 carbon Chemical pitch Industries, Ltd. (trade mark) 6-2Activated Kuraray Coconut 1584 0.79 carbon Chemical shell (KURARAY Co.,Ltd. COAL YP- 17D) 6-3 Activated Kuraray Coconut 885 0.40 carbonChemical shell (KURARAY Co., Ltd. COAL GW) 6-4 KREMEZIN Kureha Petroleum1079 0.60 (primary Corporation pitch component)

The adsorbed amounts (g) of creatinine, adsorbed amounts (g) of alizarincyanine green, adsorbed amounts of lysozyme (g) and adsorbed amounts ofalbumin (g) per gram of the porous carbon material or activated carbonare shown below in Table 8, Table 9, Table 10 and Table 11.

Adsorbed amounts (g) of creatinine per gram of the porous carbonmaterial or the like

Number average molecular weight of creatinine: 131

TABLE 8 Specific Pore Adsorbed amount of surface volume creatinine (g)area (m²/g) (cm³/g) Ex. 6-1 9.41 589 0.60 Ex. 6-3 9.13 727 0.63 Ex. 6-418.01 836 0.66 Ex. 6-5 28.62 930 0.80 Ref. Ex. 6-1 43.31 1231 0.57 Ref.Ex. 6-2 28.00 1584 0.79 Ref. Ex. 6-3 12.18 885 0.40 Ref. Ex. 6-4 16.881079 0.60

Adsorbed amounts (g) of alizarin cyanine green per gram of the porouscarbon material or the like

Number average molecular weight of alizarin cyanine green: 623

TABLE 9 Adsorbed amount of Specific Pore alizarin cyanine surface volumegreen (g) area (m²/g) (cm³/g) Ex. 6-1 104.18 589 0.60 Ex. 6-3 162.26 7270.63 Ex. 6-4 177.18 836 0.66 Ex. 6-5 201.18 930 0.80 Ref. Ex. 6-1 82.821231 0.57 Ref. Ex. 6-3 47.62 885 0.40 Ref. Ex. 6-4 56.42 1079 0.60

Adsorbed amounts (g) of lysozyme per gram of the porous carbon materialor the like

Number average molecular weight of lysozyme: 14,307

TABLE 10 Specific Pore Adsorbed amount of surface volume lysozyme (g)area (m²/g) (cm³/g) Ex. 6-1 214.36 589 0.60 Ex. 6-2 488.96 951 1.68 Ex.6-3 179.92 727 0.63 Ex. 6-4 179.57 836 0.66 Ex. 6-5 190.23 930 0.80 Ref.Ex. 6-1 84.02 1231 0.57 Ref. Ex. 6-2 109.54 1584 0.79 Ref. Ex. 6-3 91.66885 0.40 Ref. Ex. 6-4 58.23 1079 0.60

Adsorbed amounts (g) of albumin per gram of the porous carbon materialor the like

Number average molecular weight of albumin: 66,000

TABLE 11 Specific Pore Adsorbed amount of surface volume albumin (g)area (m²/g) (cm³/g) Ex. 6-1 524.21 589 0.60 Ex. 6-2 516.95 951 1.68 Ex.6-3 95.09 727 0.63 Ex. 6-4 313.54 836 0.66 Ex. 6-5 136.96 930 0.80 Ref.Ex. 6-1 385.41 1231 0.57 Ref. Ex. 6-2 181.71 1584 0.79 Ref. Ex. 6-3 1.39885 0.40 Ref. Ex. 6-4 475.93 1079 0.60

It was recognized from Table 8 that in each of Example 6-1, Example 6-3,Example 6-4 and Example 6-5, the adsorbed amount of creatinine per gramof the porous carbon material tended to increase with the value ofspecific surface area and the value of pore volume of the porous carbonmaterial, and moreover, a good correlation was observed therebetween. Ineach of the referential examples, on the other hand, no good correlationwas observed between the adsorbed amount of creatinine per gram of theactivated carbon and the value of specific area and the value of porevolume of the activated carbon probably due to the difference in theproduction procedure.

It was also recognized from Table 9 that in each of Example 6-1, Example6-3, Example 6-4 and Example 6-5, the adsorbed amount of alizarincyanine green per gram of the porous carbon material tended to increasewith the value of specific surface area and the value of pore volume ofthe porous carbon material, and moreover, a good correlation wasobserved therebetween. Further, the examples showed greater adsorbedamounts than the referential examples.

In addition, from Table 10, the adsorbed amount of lysozyme per gram ofthe porous carbon material remained substantially at a constant valuewithout depending much on the value of specific surface area and thevalue of pore volume of the porous carbon material in Example 6-1,Example 6-3, Example 6-4 and Example 6-5. Further, the examples showedgreater adsorbed amounts than the referential examples. Example 6-2showed a significantly large adsorbed amount compared with the otherexamples and the referential examples.

Moreover, from Table 11, the adsorbed amount of albumin per gram of theporous carbon material did not depend on the value of specific surfacearea and the value of pore volume of the porous carbon material inExample 6-1, Example 6-2, Example 6-3, Example 6-4 and Example 6-5. Theporous carbon materials of Example 6-1 and Example 6-2 showedsignificantly great adsorbed amounts. Further, Example 6-1 and Example6-2 showed greater adsorbed amounts than the referential examples.

Based on the results shown in Table 8 to Table 11, the standardizedvalues of the adsorbed amount of creatinine, adsorbed amount of alizarincyanine green, adsorbed amount of lysozyme and adsorbed amount ofalbumin, said standardized values having been obtained assuming that theadsorbed amounts per gram of the primary component of KREMEZIN ofReferential Example 6-4 were “1.0,” are shown in Table 12. It isappreciated from Table 12 that the porous carbon material of Example 6effectively adsorbs organic substances the number average molecularweights of which are 1×10³ to 1×10⁴ in particular.

TABLE 12 Adsorbed amount of Adsorbed alizarin Adsorbed Adsorbed amountof cyanine amount of amount of creatinine green lysozyme albumin Number133 623 14307 66000 average molecular weight, M_(n) Ex. 6-1 0.56 1.853.68 1.10 Ex. 6-5 1.70 3.57 3.27 0.29 Ref. Ex. 6-1 2.57 1.47 1.44 0.81Ref. Ex. 6-4 1.0 1.0 1.0 1.0

As has been described above, it has been found that the moleculeadsorption characteristics of each porous carbon material differdepending on the differences of the porous carbon material in itsparameters such as specific surface area and pore volume, thedifferences of the porous carbon material in its physical surfaceconditions and chemical surface conditions, the differences of chemicalinteraction between the porous carbon material and the adsorbedmaterial, and the differences of the porous carbon material in itsproduction procedure. Further, a difference was observed especiallybetween the behavior of a porous carbon material upon adsorption ofmolecules having a small molecular weight and the behavior of the porouscarbon material upon adsorption of molecules having a large molecularweight. It has been found that compared with the activated carbons ofthe comparative examples, the porous carbon materials according to thepresent invention adsorb substances having medium molecular weights orlarge molecular weights still better. By determining the correlationsbetween the molecular weight of molecules to be adsorbed and theparameters, such as specific surface area and pore volume, andproduction procedure of a porous carbon material on the basis of varioustests, molecules can, therefore, be selectively adsorbed by the porouscarbon material. Therefore, the adsorbent according to the presentinvention is expected to bring about significant advantageous effects invarious medical applications which require adsorption.

Example 7

Example 7 relates to the carrier according to the present invention forcarrying a drug thereon. In Example 7, a porous carbon material [aplant-derived material as a raw material for the porous carbon materialwas the same rice husk as in Example 1 (grown in Kagoshima Prefecture,rice husk of Isehikari variety)] was used as a carrier for a drug in adrug release preparation that can appropriately control the release rateof the drug. And, the release rate of ibuprofen (a non-steroidalanti-inflammatory drug, NSAID) was measured.

Specifically, the porous carbon material (0.10 g) obtained by the sameprocedure as in Example 1 (except that the carbonization temperature andcarbonization time were set at 800° C. and 1 hour, respectively) wasimmersed overnight in a 0.10 g:10 mL solution of ibuprofen and hexane.Subsequently, filtration was conducted by a membrane filter, followed byvacuum drying at 40° C. The resulting complex of the porous carbonmaterial and ibuprofen was mixed in a phosphate buffer (pH 7.3; 40 mL),and the concentrations of ibuprofen at respective times were measured byultraviolet spectroscopy and were then calculated. It is to be notedthat the value of specific surface area and the value of pore volume ofthe porous carbon material of Example 7 are as shown under Example 6-1in Table 6.

Using the activated carbon (0.10 g) of Referential Example 6-1 shown inTable 7, an activated carbon-ibuprofen complex was obtained by the sameprocedure as in Example 7. As Comparative Example 7, the resultingactivated carbon-ibuprofen complex was mixed in a phosphate buffer (pH7.3; 40 mL), and the concentrations of ibuprofen at respective timeswere measured by ultraviolet spectroscopy and were then calculated. Itis to be noted that the value of specific surface area and the value ofpore volume of the activated carbon are as shown below.

Porous carbon Activated carbon material of of Comparative Example 7Example 7 Specific surface area: 589 m²/g 1321 m²/g Pore volume: 0.60cm³/g 0.57 cm³/g

The measurement results of the concentrations of ibuprofen at respectivetimes are shown in FIG. 10. It is appreciated that compared withComparative Example 7, the released amount of ibuprofen was large inExample 7.

Example 8

Example 8 relates to the mask and adsorbing sheet according to thepresent invention. In Example 8, a porous carbon material [aplant-derived material as a raw material for the porous carbon materialwas the same rice husk as in Example 1 (grown in Kagoshima Prefecture,rice husk of Isehikari variety)] was applied as an adsorbent to ananti-pollinosis mask. A schematic of the anti-pollinosis mask is shownin FIG. (A) of FIG. 11, and the schematic cross-sectional structure of amain part (adsorbing sheet) of the anti-pollinosis mask is illustratedin FIG. (B) of FIG. 11. The main part of the anti-pollinosis mask has astructure that the porous carbon material in the form of a sheet is heldbetween a nonwoven fabric and another nonwoven fabric both of which aremade of cellulose. To prepare the porous carbon material of Example 1 inthe form of the sheet, it is possible to adopt, for example, such amethod that forms a carbon-polymer complex with carboxynitrocelluosebeing used as a binder. On the other hand, the adsorbing sheet ofExample 8 is formed of a sheet-shaped member, which is made of theporous carbon material produced from the same rice husk as in Example 1(specifically, a carbon-polymer complex containing carboxynitrocelluloseas a binder), and a support member supporting the sheet-shaped memberthereon (specifically, nonwoven fabrics which as a support member, holdthe sheet-shaped member therebetween). By applying the porous carbonmaterial according to the present invention as an adsorbent to variousmasks such as an anti-pollinosis mask, the masks are considered to beable to effectively adsorb pollen, for example, by adsorbing the pollenat protein sites thereof on the porous carbon material.

The present invention has been described above based on the preferredexamples. The present invention is, however, not limited to theseexamples, and various modifications are feasible. In the examples, thedescriptions were made about the cases each of which employed rice husk,straw, reed or seaweed stem as a raw material for the porous carbonmaterial. However, other plants may also be employed as raw materials.Examples of such other plants can include vascular plants, ferns andmosses which grow on land, algae, seaweeds, and the like. They may beused singly or plural ones of them may be used in combination.

Concerning the porous carbon material according to the presentinvention, adequate ranges have also been described as to its specificsurface area based on the nitrogen BET method and the contents ofvarious elements. The description, however, does not completely negatethe possibility that the value of specific surface area or the contentsof various elements may fall outside the above-described ranges. Inother words, the above-described adequate ranges are absolutely theranges which are particularly preferred to bring about the advantageouseffects of the present invention. The value of specific surface area orthe like may, therefore, somewhat depart from the above-described rangeinsofar as the advantageous effects of the present invention can beobtained.

Further, the description has been made taking, as an example, a lithiumion secondary battery as one application example of the porous carbonmaterial according to the present invention. The application of theporous carbon material according to the present invention is, however,not necessarily limited to batteries, and can be applied toelectrochemical devices other than batteries. For example, electricdouble layer capacitors and the like can be mentioned. The descriptionhas also been made about the cases in which lithium was used as anelectrode reactant. However, other Group-1A elements such as sodium andpotassium and Group-2A elements such as magnesium and calcium in theshort-form periodic table, and light metals such as aluminum may also beused. In such cases, the porous carbon material according to the presentinvention can also be used as an anode active material.

In addition, the porous carbon material described in Example 6-1 can beapplied to Example 5 and Example 8, the porous carbon materialsdescribed in Example 6-2, Example 6-3, Example 6-4 and Example 6-5 andsubjected to the respective activation treatments can be applied toExample 5, Example 7 and Example 8, the porous carbon materialsdescribed in Example 1 to Example 4 can be applied to Example 6 andExample 7, and the porous carbon materials described in Example 2 toExample 4 can be applied to Example 8.

In the plant-derived material for the porous carbon material accordingto the present invention, silicon is contained at 5 wt % or more. Byconducting carbonization at a temperature in the range of from 800° C.to 1,400° C. upon converting the plant-derived material into a porouscarbon material precursor or a carbonaceous material throughcarbonization in such a temperature range, the silicon contained in theplant-derived material is not converted into silicon carbide (SiC) butis converted into silicon components (oxidized silicon compounds) suchas silicon dioxide (SiO_(x)), silicon oxide and silicon oxide salts. Asa result of elimination of the silicon components (the oxidized siliconcompounds) such as the silicon dioxide, the silicon oxide and thesilicon oxide salts in the next step by the treatment with the acid, oralkali(base), a large value of specific surface area as determined bythe nitrogen BET method can be obtained accordingly. Further, thecarbonization at a temperature in this range provides non-graphitizablecarbon, thereby making it possible to obtain excellent cyclingcharacteristics when employed in electrochemical devices.

In the porous carbon material according to the present invention, thevalue of specific surface area as determined by the nitrogen BET methodis 10 m²/g or more, the content of silicon is 1 wt % or lower, and thevolume of pores as determined by the BJH method or MP method is 0.1 cm³or more. Excellent functionality is therefore available. As a result,excellent characteristics can be obtained when the porous carbonmaterial according to the present invention is used in anelectrochemical device such as, for example, a battery like a lithiumion secondary battery (nonaqueous electrolyte secondary battery) or anelectric double layer capacitor. According to the process of the presentinvention for the production of the porous carbon material, on the otherhand, the porous carbon material is also provided with excellentfunctionality. When employed, for example, in an electrochemical devicesuch as a battery like a lithium ion secondary battery (nonaqueouselectrolyte secondary battery) or an electric double layer capacitor,superb characteristics can be obtained. In addition, the porous carbonmaterial according to the present invention is optimal, for example, asa porous carbon material for orally-administrable adsorbent preparation,also as a porous carbon material intended to adsorb proteins or viruses,also as a porous carbon material which composes the drug releasepreparation capable of appropriately controlling the drug release rate,and also as an adsorbent in masks and as an adsorbent in adsorbingsheets.

The invention claimed is:
 1. A porous carbon material comprising amaterial obtained from carbonization of a raw material including ricehusk, the raw material having a silicon content of at least 5 wt %, theraw material is heat treated before carbonization, and the raw materialis treated by an alkali treatment after carbonization to reduce thesilicon content, the porous carbon material having a value of specificsurface area of at least 10 m²/g as measured by the nitrogen BET method,a pore volume of at least 0.1 cm³/g as measured by the BJH method and MPmethod, and an R value of 1.5 or greater, wherein the porous carbonmaterial includes mesopores having pore sizes from 2 nm to 50 nm andobtained from the alkali treatment of the raw material aftercarbonization, the porous carbon material further includes macroporesand micropores, the R value is expressed as R=B/A, the A referring to anintensity at an intersection between the baseline of a diffraction peakof the (002) plane as obtained based on powdery X-ray diffractometry ofthe porous carbon material and a perpendicular line downwardly drawnfrom the diffraction peak of the (002) plane, and the B referring to theintensity of the diffraction peak of the (002) plane.
 2. The porouscarbon material according to claim 1, wherein the content of magnesiumis at least 0.01 wt % but at most 3 wt %, the content of potassium is atleast 0.01 wt % but at most 3 wt %, and the content of calcium is atleast 0.05 wt % but at most 3 wt %.
 3. The porous carbon materialaccording to claim 1, wherein the material has a value of specificsurface area of at least 50 m²/g as measured by the nitrogen BET method.4. The porous carbon material according to claim 1, wherein the materialhas a value of specific surface area of at least 100 m²/g as measured bythe nitrogen BET method.
 5. The porous carbon material according toclaim 1, wherein the material has a value of specific surface area of atleast 400 m²/g as measured by the nitrogen BET method.
 6. The porouscarbon material according to claim 1, wherein the value of specificsurface area of the porous carbon material is up to 1500 m²/g asmeasured by the nitrogen BET method.
 7. The porous carbon materialaccording to claim 1, wherein the material is further washed after thealkali treatment.
 8. The porous carbon material according to claim 1,wherein the raw material is heat treated before carbonization in asubstantially oxygen-free state.
 9. The porous carbon material accordingto claim 1, wherein the alkali treatment includes treatment with NaOH.10. An adsorbent comprising the porous carbon material of claim
 1. 11.The adsorbent according to claim 10, wherein the adsorbent is capable ofadsorbing creatinine, alizarin cyanine green, lysozyme, albumin, anorganic substance having a number average molecular weight of 1×10³ to1×10⁴, and combinations thereof.
 12. A mask provided with an adsorbentcomprising the porous carbon material of claim
 1. 13. An adsorbingsheet, comprising a sheet-shaped member including the porous carbonmaterial of claim
 1. 14. A carrier for carrying a drug, the carriercomprising the porous carbon material of claim
 1. 15. A porous carbonmaterial comprising a material obtained from carbonization of a rawmaterial selected from the group consisting of grain husk, straw of riceplant, barley, wheat, and mixtures thereof, the raw material having asilicon content of at least 5 wt %, the raw material is heat treatedbefore carbonization, and the raw material is treated by an alkalitreatment after carbonization to reduce the silicon content, the porouscarbon material having a value of specific surface area of at least 10m²/g as measured by the nitrogen BET method, a pore volume of at least0.1 cm³/g as measured by the BJH method and MP method, and an R value of1.5 or greater, wherein the porous carbon material includes mesoporeshaving pore sizes from 2 nm to 50 nm and obtained from the alkalitreatment of the raw material after carbonization, the porous carbonmaterial further includes macropores and micropores, the R value isexpressed as R=B/A, the A referring to an intensity at an intersectionbetween the baseline of a diffraction peak of the (002) plane asobtained based on powdery X-ray diffractometry of the porous carbonmaterial and a perpendicular line downwardly drawn from the diffractionpeak of the (002) plane, and the B referring to the intensity of thediffraction peak of the (002) plane.
 16. A porous carbon materialcomprising a material obtained from carbonization of a raw materialcomprising grain husk, the raw material having a silicon content of atleast 5 wt %, the raw material is heat treated before carbonization, andthe raw material is treated by an alkali treatment after carbonizationto reduce the silicon content, the porous carbon material having a valueof specific surface area of at least 10 m²/g as measured by the nitrogenBET method, a pore volume of at least 0.1 cm³/g as measured by the BJHmethod and MP method, and an R value of 1.5 or greater, wherein theporous carbon material includes mesopores having pore sizes from 2 nm to50 nm and obtained from the alkali treatment of the raw material aftercarbonization, the porous carbon material further includes macroporesand micropores, the R value is expressed as R=B/A, the A referring to anintensity at an intersection between the baseline of a diffraction peakof the (002) plane as obtained based on powdery X-ray diffractometry ofthe porous carbon material and a perpendicular line downwardly drawnfrom the diffraction peak of the (002) plane, and the B referring to theintensity of the diffraction peak of the (002) plane.